It is common to provide metal pads on a chip, such as a semiconductor chip or a ceramic submount, then deposit solder bumps on the pads. The pads may have different heights due to a varying thickness of the chip or due to the pads having varying thicknesses. The chip is then positioned over a larger substrate containing corresponding metal pads, and the solder bumps are reflowed to bond the opposing pads together to form electrical connections between the chip and the substrate.
Solder bumps may have heights between tens of microns to hundreds of microns. These dimensions are typically many times greater than the differences in heights of the pads on the chip so, during reflow, there is a sufficient volume of solder on each pad to bridge across any gap to an opposing pad on the substrate. Additionally, the sizes of the solder bumps themselves vary somewhat, and the relatively large volume of solder is sufficient to bridge across any gap to an opposing pad on the substrate.
However, for very small chip pads or for pads that are very close together, less solder must be used. There is a point where the small volume of solder on each pad is not sufficient to bridge a gap between a solder bump and the opposing pad during reflow due to the difference in heights of the pads on the chip being too great. Therefore, the size of the pads and/or their pitch is limited.
FIGS. 1A-1C and FIGS. 2A and 2B illustrate the problem with an insufficient size solder bump used with pads having differences in heights.
In FIG. 1A, a chip 10 is shown that may be a semiconductor chip, a ceramic submount, or other chip that may benefit from small solder bumps. The chip 10 has a small metal solder pad 12 and a small solder area 14 lower in height than the solder pad 12. The chip 10 may have an array of such pads 12 or areas 14. The area 14 represents any solder attachment area (which may also be another metal pad) that has a lower height than the pad 12. Small solder bumps 16 and 18 are deposited over the pad 12 and area 14 and somewhat spread out over the surface. It is assumed the volumes of the solder bumps 16 and 18 are the same. If the pad 12 or area 14 were larger, the solder bumps 16 and 18 may resemble a sphere due to the surface tension of the solder.
The solder bumps in all embodiments may be conventional such as tin, lead, silver, gold, nickel, other metals and alloys thereof.
In FIG. 1B, the chip 10 is positioned over a substrate 20. Note how the solder bump 16 is pressed against the opposing metal pad 22 on the substrate 20, but the solder bump 18 is slightly separated from the opposing pad 24 on the substrate 20.
In FIG. 1C, the structure of FIG. 1B is heated to reflow the solder. The solder bump 16 forms a good bond between the pads 16 and 22, but the solder bump 18 separates into two portions 18A and 18B and forms an unreliable connection between the pad 24 and the area 14. Therefore, more solder is needed and the solder pads/areas need to be larger or more separated. Thus, the pad/area height difference limits the size and/or density of the solder pads/areas.
In FIG. 2A, a chip 26 has a pad 28 and a lower height solder area 30, with a dielectric portion 32 between the pad 28 and area 30. Recessed solder bumps 34 and 36, of the same volume solder, are formed on the pad 28 and area 30
In FIG. 2B, the chip 26 is positioned over the substrate 20. Note how the solder bump 34 is pressed against the opposing metal pad 22 on the substrate 20, but the solder bump 36 is slightly separated from the opposing pad 24 on the substrate 20.
During reflow or an ultrasonic bonding process, the solder hump 36 will not make a good connection to the opposing pad 24.
What is needed is a technique that improves the reliability of connections made using solder bumps where the heights of the solder pads or solder areas on a chip are different.